Polyester is used for various purposes such as electric insulation, an optical application, and the like. In recent years, particularly, attention has been paid in respect of application of polyester for electric insulation to solar cells in a form of a back sheet for solar cells (hereinafter, it may be simply referred to as a “back sheet”) and the like.
Generally, a solar cell module is used in an environment such as an outdoor environment in which the module is continuously exposed to rain and wind. Accordingly, durability of polyester having a back sheet is one of the important issues.
In order to obtain the durability of polyester in a humid and hot environment, it is important to avoid intrusion of moisture into a substrate at a side of a solar cell that is caused when an encapsulant adjacent to a back sheet is separated from the back sheet or when layers inside a back sheet having a layered structure are separated from each other.
It has been disclosed a technique in which a urethane resin containing constituent components including an aliphatic polycarbonate polyol is used for a coating layer of a back sheet, and the absorbance of the constituents is regulated so as to improve the adhesiveness between the encapsulant and the back sheet (for example, see Japanese Patent Application Laid-Open (JP-A) No. 2011-139036).
It has been disclosed a technique in which an inorganic oxide-deposited film is formed in advance as a pre-treated surface layer on a surface of a substrate film by a CVD method, a PVD method, or the like, so as to improve the adhesiveness between the substrate film and the deposited film and/or a laminate film (for example, see Japanese Patent No. 4217935).
It has been disclosed an attempt at ameliorating the dimensional stability of a polyester film based on the infrared spectroscopic characteristics of polyester (for example, see JP-A No. 2000-071405 and JP-A No. 2000-302892).